You must have a PayPal Business Account to get a client ID. If you are signing up for the first time, follow the prompts to create a Business Account. Otherwise, follow Step 2 to upgrade your account
Step 2: On your Developer Dashboard, make sure you are on the "My Apps & Credentials" tab. Click the "Live" button to switch out of the Sandbox.
If you do not have a Business Account, you will see the screen below. Click the "Upgrade Account" link and follow the prompts
Step 3: Click on Create App
Step 4: Enter an App Name (this can be any name) and then click the Create App button
Step 5: Copy your Client ID
Step 6: Paste your Client ID into the Paypal Client ID field in your Showroom Settings.
Step 7: If your ID is valid, you should see a Paypal button appear and you are ready to start selling your artwork with PayPal.